1. Field of the Invention
The invention relates to a method of scrambling a sound signal applied to a transmission channel by providing several modes of alteration of the sound signal, the change-over from one mode to the other being triggered by a control signal, and a system for scrambling/descrambling the sound, comprising a sound signal source, a transmitter using a transmission channel and receivers, with a device at the transmitter end which is capable of altering the sound signal in several ways so that an unadapted receiver reproduces an unintelligible sound.
2. Description of the Prior Art
Sound scrambling is a process notably used in radio broadcasting or television broadcasting systems for the purpose of reserving the reception for a selected audience.
A sound scrambling method in a television system is known from the document DE-OS-1,907,580. According to this method three modes of sound transmission are provided: the normal mode in the unaltered baseband, a mode in which the band is shifted and not reversed, and a mode in which the band is shifted and reversed which can be realized by means of a modulator having a modulating frequency above the sound passband. The change-over from one mode to the other is controlled in this system at the instant of a field flyback in accordance with a pseudo-random process which is common for the sound and the image.
This method has the drawback that the mode changes may involve clicks in the sound because there is no correlation between the sound and the instants of changing the mode. Moreover, there are only two modes with an alteration which provides a limited protection of secret information.